Preparation and properties of thyroid cell membranes

Localization of 3',5'-cyclic adenosine monophosphate phosphodiesterase (cAMP-PDEase) activity in isolated bovine thyroid plasma membranes

Isolated bovine thyroid plasma membrane preparations were obtained by isopycnic density gradient centrifugation. Cyclic AMP-PDEase (EC 3.1.4.c) activity has been demonstrated by electron microscopic histochemistry on the unit membrane of isolated bovine thyroid cells. 3-isobutyl-1-methyl-xanthine (IBMX) produced partial inhibition, while omission of the substrate revealed no reaction product deposition. These observations correlated well with biochemical studies that showed 0.4% of the total cAMP-PDEase activity to be present in the plasma membrane preparations. Kinetic analysis of cAMP hydrolysis yielded two apparent Michaelis constants for the homogenate and the plasma membrane-rich fraction. Dose-response curves for IBMX inhbition showed cAMP-PDEase of the homogenate to be more sensitive to inhibition than that of the plasma membrane-bound enzyme. Furthermore, wash experiments indicate that the plasma membrane-associated enzyme is tightly bound. This investigation strengthens our previous study and suggests that bovine thyroid cell plasma membranes contain a cAMP-PDEase that may be involved in interactions between the cell and the external environment in a manner yet to be determined.

The subcellular biochemistry of thyroid

In this review the subcellular localization of enzymes and constituents in thyroid is discussed. Conditions and results of differential pelleting and gradient centrifugation studies are described with special attention to the validity of the markets used (Table VI). Special approaches to the isolation and characterization of thyroid organelles and membranes are extensively reviewed (Table VII). Subcellular fractionation of thyroid tissue has been shown to be an arduous task. Classic approaches for differential pelleting and gradient centrifugation, which have been proved successful for rat liver, are not always equally satisfactory for thyroid. The major problem is the toughness of the tissue requiring rather traumatizing homogenizing procedures. Nevertheless, the fractionation procedures did allow the subcellular localization of some enzymes and constituents to be established with a high degree of certainty. Furthermore, enriched subcellular fractions have been isolated which have been useful for biochemical studies concerning the specific function of this tissue.

Evidence for a PGE-receptor in the rat kidney

A membrane preparation derived from homogenates of the rat kidney has been shown to possess a high affinity for prostaglandins of the E-series. Other prostaglandins including PGI2 had characteristic but significantly weaker binding properties. A 15-hydroxyprostaglandin dehydrogenase (PGDH) was found to be associated with the membrane fraction studied. However it was possible to distinguish between this and the "receptor" binding by kinetic studies and by the use of a new inhibitor highly specific for PGDH.